The present invention relates to a high-speed, large-capacity AC motor drive apparatus which is utilized for an iron-steel rolling mill, a water pump, a tunnel evacuation blower, or the like.
Motors can be primarily classified into DC motors and AC motors. The former produces small torque ripples and good controllability, and can be easily handled. As a result, DC motors have been used in a wide field of applications. DC motors do, however, require cumbersome maintenance for their brushes and commutators, and there are limitations on their maximum operating speed and/or maximum capacity. Therefore, DC motors have tended to be replaced by AC variable-speed motors.
Typical AC motors include induction motors and synchronous motors. Although AC motors also include reluctance motors and hysteresis motors, they have a considerably narrower field of applications.
A commutatorless motor is known, in which a counterelectromotive force of a synchronous motor is used to naturally commutate a thyristor inverter. Since the commutatorless motor utilizes natural commutation, it can easily have a large capacity, has similar controllability to that of the DC motors, and can be used in a wide range of applications. However, since the commutatorless motor requires a field pole, the overall motor device becomes bulky, and has a small overload strength, due to the limitations on natural commutation.
An inductance motor, in particular, a squirrel-cage inducation motor, has a simple structure, is rigid, and can be easily handled. However, this motor requires a self-excited inverter, and a converter, used together with the motor, is subjected to certain limitations.
Nowadays, self-extinction elements such as transistors, GTOs, and the like tend to have a large capacity, and are used in the self-excited inverter. In particular, a pulse-width modulation (PWM) controlled inverter can supply a sine wave current to a motor. Therefore, an AC variable-speed motor of a low noise and producing small torque ripples can be realized. Meanwhile, various control techniques, such as V/f=constant control, slip frequency control, vector control, and the like are available, and enable characteristics equivalent to those of the DC motors to be obtained.
A cycloconverter is known as a typical example which utilizes a voltage from an AC power source to effect natural commutation. The cycloconverter can supply a sine wave current to a motor, and its capacity can easily be increased by means of natural commutation. In particular, a reactive-power compensation type cycloconverter, in which an input power factor at a receiving end is controlled to be always 1, has received a great deal of attention (cf. U.S. Pat. No. 4,418,380 issued on Nov. 29, 1983; U. S. Pat. No. 4,570,214 issued on Feb. 11, 1983; or Japanese Patent Publication No. 59-14988).
The conventional AC motor drive apparatus has been utilized in a wide variety of fields while taking its advantage. However, an apparatus for driving a large-capacity, high-speed motor cannot be easily realized by means of the above-mentioned conventional techniques. More specifically, although the cycloconverter utilized natural commutation so that its capacity can be easily increased, since the cycloconverter has a low output frequency, it cannot be used for a high-speed operation. On the other hand, a self-excited inverter requires self-extinction elements such as transistors, GTOs, and the like. Therefore, the apparatus becomes expensive, and it is difficult to increase its capacity.
Since the commutatorless motor utilized natural commutation, its capacity can be easily increased, and high-speed operation can be easily achieved. However, the motor itself is commplicated and bulky. Further, since a rectangular current is supplied to an armature winding, torque ripples of the motor are increased. In addition, problems associated with the manner of commutation at the beginning of energization and an insufficient overload strength still remain.
On the other hand, along with an increase in the capacity of the motor, the influence of reactive power generated from the power source and that of harmonic components of the reactive power cannot be ignored. Variations in reactive power cause variations in the power source system voltage, and adversely influence other electrical equipment connected to the the same power source system. A harmonic current induces induction problems in television systems, radio receivers, or communication lines, and harmonic components of the 3rd, 5th, 7th orders cannot easily be removed.
A reactive-power compensation type cycloconverter (cf. U.S. Pat. Nos. 4,418,380 and 4,570,214) is an effective means for solving the reactive power problem, which serves as a power converter for maintaining the input power factor of the receiving end constantly at 1. However, a harmonic current, depending on the output frequency, appears at the input side, and countermeasures must be taken thereagainst.
Recently, a power converter having the functions of both an AC power converter and an active filter has been proposed (e.g., Japanese Patent Disclosure (Kokai) No. 59-61475). An AC motor driving system, constituted by a combination of this power converter and a self-excited inverter, has received a great deal of attention.
In this system, since an input current is controlled to be a sine wave in the same phase as that of the power source voltage, a harmonic component is small, and the input power factor can be maintained to be always 1. However, the converter must be constituted by self-extinction elements such as transistors and GTOs. Therefore, a large-capacity system is difficult to realize and has an economical problem.